Validation missing against external tables

[tvogels01]

Summary

When a view depends on a table inside a schema that's marked as an "external" one in the configuration,
Arthur skips checking the dependencies entirely and trusts the user's configuration.

We should, however, at least check that the external tables already exist!

Details

The SVV_EXTERNAL_TABLES table allows us to pull all existing external tables. We should take the intersection with the external schemas that are marked as "external". During the validation we can then ensure that any table marked as dependency is either known because it is managed by Arthur or is in the list of external tables.